Position sensing apparatus and method

ABSTRACT

There is described an inductive position sensor in which a signal generator applies a excitation signal to an excitation winding formed on a first member, the excitation winding being electromagnetically coupled to at least two resonant circuits formed on a second member which are spaced from each other along a measurement path. The excitation windings are shaped so that the electromagnetic coupling between the exciting winding and each of the resonators varies along the measurement path. In this way, by applying an excitation signal to the excitation winding, respective different signals are induced in the resonators which depend upon the relative position of the first and second members.

This invention relates to a sensing apparatus and method, in particularfor sensing the relative position of two members.

Various forms of inductive sensor have been used to generate signalsindicative of the position of two relatively movable members. Forexample, UK Patent Application No. 2374424 describes a position sensorin which two excitation windings and a sensor winding are formed on afirst member, and a resonant circuit is formed on a second member. Thetwo excitation windings are shaped so that the electromagnetic couplingbetween the excitation windings and the resonant circuit varies along ameasurement path in accordance with a sine function and a cosinefunction respectively. By applying an in-phase oscillating signal and aquadrature oscillating signal (that is 90° out of phase with thein-phase oscillating signal) to the first and second excitation windingsrespectively, an oscillating signal is generated in the resonant circuitwhose phase is dependent upon the relative position between the firstand second members along the measurement path. The oscillating signal inthe resonant circuit in turn induces an oscillating signal in the sensorwinding whose phase is indicative of the relative position between thefirst and second members along the measurement path.

A problem with the position sensor described in GB 2374424A is that thedrive circuitry which applies the quadrature pair of excitation signalsto the two excitation windings is relatively complex.

According to an aspect of the invention, there is provided a positionsensor in which a signal generator applies an excitation signal to anexcitation winding formed on a first member, wherein the excitationwinding is electromagnetically coupled to at least two resonant circuitsformed on a second member which are spaced from each other along ameasurement path. The excitation windings are shaped so that theelectromagnetic coupling between the excitation winding and each of theresonators varies along the measurement path. In this way, by applyingan excitation signal to the excitation winding, respective differentsignals are induced in the resonators which depend upon the relativeposition of the first and second members.

An exemplary embodiment of the present invention will now be describedwith reference to the accompanying drawings in which:

FIG. 1 schematically shows a perspective view of a position sensor;

FIG. 2 schematically shows the main components of the position sensorillustrated in FIG. 1;

FIG. 3 schematically shows how the respective phases of signals inducedin two resonant circuits which form part of the position sensorillustrated in FIG. 1 vary with the frequency of a driving signal;

FIG. 4 shows an unmodulated signal and a sensed signal generated by thesensor illustrated in FIG. 1;

FIG. 5 shows part of a signal that is sensed by a sensor winding, whichforms part of the position sensor illustrated in FIG. 1, after mixingwith a second frequency;

FIG. 6 shows part of the signal illustrated in FIG. 5 after filtering,together with a reference signal; and

FIG. 7 is a schematic block diagram showing the main elements of theposition sensor illustrated in FIG. 1.

FIG. 1 schematically shows a position sensor for detecting the positionof a sensor element 1 which is slidably mounted to a support (not shown)to allow linear movement along a measurement direction (the direction Xin FIG. 1). A printed circuit board (PCB) 3 extends along themeasurement direction and has printed thereon conductive tracks whichform a sine coil 5 and a sense coil 7, each of which are connected to acontrol unit 9. A display 11 is also connected to the control unit 9 fordisplaying a number representative of the position of the sensor element1 in the measurement direction.

The sensor element 1 includes a printed circuit board 13 havingconductive tracks printed thereon which form a first resonant circuit 15a and a second resonant circuit 15 b which are spaced from each other inthe measurement direction. By applying an excitation signal to the sinecoil 3, resonant signals are induced in the two resonant circuits 15 a,15 b on the sensor element 1, and these resonant signals in turn inducea signal in the sense coil 7 which is processed by the control unit 9 todetermine the position of the sensor element 1 along the measurementdirection.

As shown in FIG. 1, the PCB 3 is generally rectangular in shape with thelengthwise axis aligned with the measurement direction and the widthwiseaxis aligned perpendicular to the measurement direction. The sine coil 5and the sense coil 7 are connected to the control unit 9 via a proximallengthwise edge 17 of the PCB 3, and extend along the length of the PCB3 to a distal lengthwise edge 19.

An overview of the operation of the position sensor illustrated in FIG.1 will now be given with reference to FIG. 2. The control unit 9includes an excitation signal generator 21 which generates anoscillating excitation signal having a frequency f₀, which in thisembodiment is 1 MHz.

The excitation signal is applied to the sine coil 5 via the proximallengthwise edge 17 of the PCB 3, which substantially corresponds to aposition value of x=0. The position value increases along the length ofthe PCB 3 from the proximal lengthwise edge 17 to the distal lengthwiseedge 19, which substantially corresponds to a position value of x=L.

As shown in FIG. 2, the sine coil 5 is formed by a conductive trackwhich generally extends around the periphery of the PCB 3 apart from across-over point halfway along the PCB 3 in the measurement direction,at which point the conductive track adjacent each widthwise edge of thePCB 3 crosses to the respective opposing widthwise edge of the PCB 3. Inthis way, effectively a first current loop 23 a and a second currentloop 23 b are formed. When a signal is applied to the sine coil 5,current flows around the first current loop 23 a and the second currentloop 23 b in opposite directions, and therefore the current flowingaround the first current loop 23 a generates a magnetic field which hasan opposite polarity to the magnetic field generated by the current flowaround the second current loop 23 b.

The lay-out of the sine coil 5 is such that the field strength of thecomponent of the magnetic field B resolved perpendicular to the PCB 3which is generated by current flowing through the sine coil 5 variesalong the measurement direction from approximately zero at the pointwhere x=0, then to a maximum value at x=L/4 (the position A as shown inFIG. 1), then back to zero at x=L/2 (the position B as shown in FIG. 1),then to a maximum value (having opposite polarity to the maximum atposition A) at x=3L/4, and then back to zero at x=L. In particular, thesine coil 5 generates a magnetic field component B perpendicular to thePCB 3 which varies according to one period of the sine function, as setout in Equation 1, $\begin{matrix}{B = {A\quad\sin\quad\left( \frac{2\quad\pi\quad x}{L} \right)}} & (1)\end{matrix}$where A is a constant.

When the excitation signal is applied to the sine coil 5, an oscillatingsignal at the same frequency is induced in each of the two resonantcircuits 15 a, 15 b on the sensor element 1, with the magnitude of theinduced signal in each resonant circuit 15 being dependent upon thestrength of the magnetic field component B resolved perpendicular to thePCB 3 at the respective position of the resonant circuit 15 along themeasurement direction. In this embodiment, the first resonant circuit 15a is separated from the second resonant circuit 15 b by a distance dwhich is equal to L/4, so that as the sensor element 1 moves along themeasurement direction the magnitude of the resonant signals induced inthe first and second resonant circuits 15 vary in quadrature. A phaselag is also introduced between the excitation signal and the inducedsignal in each resonant circuit, the amount of the phase lag beingdependent upon the relationship between the frequency of the excitationsignal and the resonant frequency of each resonant circuit 15. FIG. 3shows the variation in phase lag with the frequency of the excitationsignal for the first and second resonant circuits 15 a, 15 b. Inparticular, the curve referenced 31 a shows the variation in phase lagwith the frequency of the excitation signal for the first resonantcircuit 15 a, and the curve referenced 31 b shows the variation in phaselag with the frequency of the excitation signal for the second resonantcircuit 15 b.

As shown in FIG. 3, the resonant frequency f¹ _(res) of the firstresonant circuit 15 a is set so that at the frequency f₀ of theexcitation signal, the phase lag of the induced signal in the firstresonant circuit is 3n/4, whereas the resonant frequency f² _(res) ofthe second resonant circuit 15 b is set so that at the frequency f₀ ofthe excitation signal, the phase lag of the signal induced in the secondresonant circuit is n/4. If the position of the first resonant circuit15 a along the measurement direction is X₀, then the signals I₁, I₂induced in the first and second resonant circuits 15 a, 15 b are givenby equations 2 and 3 respectively. $\begin{matrix}{I_{1} = {B\quad\sin\quad\left( \frac{2\quad\pi\quad X_{0}}{L} \right)\quad\cos\quad\left( {{2\quad\pi\quad f_{0}t} - \frac{3\quad\pi}{4}} \right)}} & (2) \\{I_{2} = {B\quad\sin\quad\left( \frac{2\quad{\pi\left( {X_{0} + \frac{L}{4}} \right)}}{L} \right)\quad\cos\quad\left( {{2\quad\pi\quad f_{0}t} - \frac{\pi}{4}} \right)}} & (3)\end{matrix}$where B is a constant.

The induced signals I₁, I₂ in turn induce a signal S in the sense coil 7proportional to the sum of the induced signals I₁ and I₂. This sumsimplifies to the expression given in Equation 4, in which C is aconstant. $\begin{matrix}{S = {C\quad\cos\quad\left( {\frac{2\quad\pi\quad X_{0}}{L} - {2\quad\pi\quad f_{0}t} + \frac{\pi}{4}} \right)}} & (4)\end{matrix}$

In effect, the phase of the signal S rotates as the sensor element 1moves along the measurement direction.

As shown in FIG. 2, the signal S is input to a timing comparator 25which compares the timing of the signal S with the timing of a referencesignal from the excitation signal generator 21 to determine a valuerepresentative of the phase of the signal S. This value is then input toa position calculator 27. which converts the value to a position valuefor the sensor element 1, and outputs a drive signal to the display 11causing the display 11 to show the position value.

As mentioned above, in this embodiment the frequency f₀ of theoscillating excitation signal is 1 MHz. This frequency is sufficientlyhigh to induce a relatively large signal in each resonant circuit 15.FIG. 4 shows the excitation signal P together with the signal S which isinduced in the sense coil 7. In FIG. 6, the sense signal S has a phasedelay of 0.1 μs with respect to the excitation signal P. At a frequencyof 1 MHz, the phase delay will always be 1 μs or less, with the resultthat, in order to determine the position of the sensor elementaccurately, it is necessary to resolve the phase delay to a value of 1to 10 ns, which is relatively difficult. If, however, the sensed signalS is mixed with a second signal of slightly lower or higher frequency, asignal as shown in FIG. 5 is generated which contains a signal at afrequency higher than the original signal, together with a lowerfrequency “beat” signal at a frequency equal to the frequency differencebetween the sensed signal and the second signal. This signal can befiltered to remove the high frequency signal and other signals and leavethe beat sinusoidal signal 35 as shown in FIG. 6. The beat signal 35 hasa phase delay that is related to the position of the sensor element 1 sothat it may be compared with a reference signal 37 of the same frequencyto determine the position of the sensor element 1. It can be seen fromFIG. 6 that the phase delay of the resulting beat signal corresponds tomuch longer times with the result that relatively inexpensive circuitrycan be employed to measure the phase delay.

FIG. 7 schematically shows the circuitry within the control unit 9 inmore detail, together with the sine coil 5 and sense coil 7. Thecircuitry within the control unit 9 comprises a microprocessor 41,signal generator 42, analogue driving circuitry 40 and analogue signalprocessing components 44.

The microprocessor 41 includes a square wave oscillator 112 whichgenerates a square wave signal at twice the frequency f₀ (i.e. at 2MHz). This square wave signal is output from the microprocessor 41 tothe signal generator 42 which divides the square wave signal by two andforms an in-phase digital signal +I at the frequency f₀. The in-phasesignal +I is sent to the analogue driving circuitry 40, and is input toa coil driver 83 which amplifies the signal and outputs the excitationsignal to the sine coil 5.

The digital generation of the excitation signals applied to the sinecoil 5 introduces high frequency harmonic noise. However, the coildriver 83 removes some of this high frequency harmonic noise, as doesthe frequency response characteristics of the sine coil 5. Further, theresonant circuits 15 within the sensor element 1 do not respond tosignals which are greatly above their respective resonant frequenciesand therefore the resonant circuits 15 also filter out a portion of theunwanted high frequency harmonic noise.

The signal S induced in the sense coil 7 is passed through a high passfilter amplifier 93 which both amplifies the received signal, andremoves low frequency noise (e.g. from a 50 Hz mains electricity supply)and any D.C. offset. The amplified signal is then input to a mixer 95,where the amplified signal is mixed with a reference signal at a secondfrequency f₁. The second signal of frequency f₁ is a digital signalhaving sinusoidal characteristics, and is generated by the signalgenerator 42. The second signal has a fundamental frequency somewhathigher or lower than that of the original signals at frequency at f₀ sothat the signal output by the mixer 95 includes components atfrequencies f₀+f₁ and at f₀−f₁. This mixed signal is then input to a lowpass amplifier filter 97 to filter out the high frequency components,i.e. those components at a frequency of f₀+f₁.

The second signal typically has a frequency f₁ that differs from f₀ bynot more than 10% of the original frequency f₀ so that the components ofthe resulting signal have a frequency f₀−f₁ which is at a much lowerfrequency than any other component of the signal and the higherfrequency components can therefore easily be removed by means of ananalogue filter. The filtered signal is then input to a band pass filteramplifier 99 having a pass band centred at f₀−f₁, after which agenerally sinusoidal third signal is formed as shown in FIG. 6.

The signal output by the band pass filter amplifier 99 is input to acomparator 101 which converts it to a square wave signal whose risingand falling edges correspond with the zero crossing points of thesinusoidal signal of FIG. 6. The square wave signal is input into atimer 104, forming part of the microprocessor 41, together with anothersquare wave signal V_(ref), generated by the signal generator 42, of thesame frequency which provides a reference phase.

The timer measures the difference between the timings of the rising andfalling edges of the signal output by the comparator 101 and thereference signal V_(ref), and outputs the measured timings to aprocessing unit 108 which determines the corresponding position valueusing a look-up table. The processing unit 108 then outputs thedetermined position value to a display controller 110 which generatesdrive signals to cause the display 11 to show the determined positionvalue.

Further details of the components and operation of the control unit 9may be found in UK patent application no. 0224100.8, whose contents arehereby incorporated by reference.

MODIFICATIONS AND FURTHER EMBODIMENTS

In the illustrated embodiment, an excitation winding (i.e. the sine coil5) is electromagnetically coupled to two resonators (i.e. the resonantcircuits 15), and the resonant signals induced in the resonant circuits15 are analysed using a sensor winding (i.e. the sense coil 7) which iselectromagnetically coupled with the two resonators. It is not essentialto use such a sensor winding because the resonant signals induced in thetwo resonators could be measured directly. Such direct measurement isnot, however, preferred because it would require electrical connectionsto be made to the sensor element.

In the illustrated embodiment, the resonant circuits 15 on the sensorelement 1 have overlapping, but not identical, ranges of frequenciesover which a sinusoidal signal applied to the sine coil 5 induces aresonant signal in the sense coil 7. The frequency of the excitationsignal is selected so that there is a quarter of a cycle phasedifference between the signals induced in the first and second resonantcircuits caused by the phase shifts which are inherent to resonatorsaround the resonant frequency.

In the illustrated embodiment, the sensor element includes two resonantcircuits 15 which are separated by a distance corresponding to a quarterof a cycle of the sine coil 5. This is not, however, essential as thesensor element could, for example, have two resonant circuits separatedby three-quarters of a cycle of the sine coil 5. Alternatively, thesensor element could have three or more spaced resonant circuits.

In the illustrated embodiment, the sine coil 7 is arranged so that themagnetic field component perpendicular to the PCB 3 varies sinusoidallyin accordance with position along the measurement direction, and the tworesonant circuits are separated by a distance of L/4 along themeasurement direction. In this way, the electromagnetic coupling betweenthe sine coil 5 and the first resonant circuit 15 a varies in accordancewith a first function (i.e. the sine function) and the electromagneticcoupling between the sine coil 5 and the second resonant circuit 15 bvaries in accordance with a second function (i.e. the cosine function).In order to achieve this, the sine coil has an alternate twisted loopstructure. However, it would be apparent to a person skilled in the artthat an enormous variety of different excitation winding geometriescould be employed to form transmit aerials which achieve the objectiveof causing the relative strengths of the resonant signals appearing inthe first and second resonant circuits to depend upon the position ofthe sensor element in the measurement direction according to respectivefirst and second functions.

In the above described embodiment, a passive resonator is used. However,in some circumstances it may be advantageous to use a resonatorincluding an amplifier so that the signal induced in the resonator isincreased.

In the illustrated embodiment, instead of detecting the phase of thesense signal, it is also possible to perform parallel synchronousdetection of the sense signal, with one synchronous detection using anin-phase signal (with respect to the excitation signal) and the othersynchronous detection using a quadrature signal (with respect to theexcitation signal). By then performing an arctangent operation on theratio of the magnitudes of the synchronously detected signals, a valuerepresentative of the position of the sensor element 1 in themeasurement direction can be obtained.

In the described embodiment, the inductive sensor is used to measure thelinear position of a first member (i.e. the sensor element 1) relativeto a second member (i.e. the PCB 3) along a rectilinear measurementpath.

Alternatively, the inductive sensor could be adapted to measure linearposition along a curved measurement path, for example a circle (i.e. arotary position sensor), by varying the layout of the sine coil in amanner which would be apparent to a person skilled in the art. Theinductive sensor could also be used as a speed detector by taking aseries of measurements of the position of the first member relative tothe second member at known timings.

In the illustrated embodiment, the sine coil, sense coil and resonantcircuits are formed by conductive tracks on a printed circuit board.Alternatively, a different planar substrate could be used. Further, thesine coil and sense coil could, if sufficiently rigid, be fixed relativeto a first member and the resonant circuits fixed relative to a secondmember without the use of a substrate. It is also not essential that thesine coil, sense coil and resonant circuits are planar because, forexample, cylindrical windings could also be used with the sensor elementmoving along the cylindrical axis of the cylindrical winding.

Of course, as the position sensor detects the relative position betweenfirst and second members, it does not matter which of the first memberand the second member are moved, or even if both are moved.

In the above described embodiment, the excitation signal is a digitalrepresentation of a sinusoidal signal. This is not essential and may beconvenient to use an excitation signal which is more easily generated.For example, the excitation signal could be a digital representation ofa triangular waveform. The phase of the sensed signal can be decoded inthe same way as the illustrated embodiment by only looking at thefundamental frequency of the sensed signal, i.e. by filtering out thehigher harmonics present in the triangular waveform. As describedpreviously, the frequency responses of the analogue driving circuitry,the sine coil and the resonant circuits are effective in removing alarge proportion of the higher harmonics.

1. A position sensor comprising: first and second members which aremovable relative to each other along a measurement path, the firstmember comprising an excitation winding and the second member comprisingfirst and second resonators spaced apart along the measurement path; anexcitation signal generator operable to generate an excitation signaland to apply the excitation signal to the excitation winding to induce afirst resonant signal in the first resonator and a second resonantsignal in the second resonator; and an analyser operable to analyse thefirst and second resonant signals to determine a value representative ofthe relative position along the measurement path of the first and secondmembers, wherein the excitation winding and the first resonator have afirst electromagnetic coupling which varies with the relative positionalong the measurement path of the first and second members in accordancewith a first function, and the excitation winding and the secondresonator have a second electromagnetic coupling which varies with saidrelative position in accordance with a second function different fromthe first function, and wherein the first resonator is operable tointroduce a first phase shift into the first resonant signal and thesecond resonator is operable to introduce a second phase shift, which isdifferent from the first phase shift, into the second resonant signal.2. The position sensor according to claim 1, wherein said analysercomprises a sensor winding electromagnetically coupled to the first andsecond resonators, wherein in response to the excitation signal beingapplied to the excitation winding, there is generated in the sensorwinding an electric signal corresponding to a combination of the firstand second resonant signals weighted in accordance with the relativeposition of the first and second members along the measurement path; anda signal processor operable to process the electric signal generated inthe sensor winding to determine a value representative of the relativeposition along the measurement path of the first and second members. 3.The position sensor according to claim 1, wherein the excitation windingand the first and second resonators are arranged so that said first andsecond functions vary sinusoidally with position with the same periodbut are out of phase with each other.
 4. The position sensor accordingto claim 3, wherein the first and second functions are one quarter of acycle out of phase with each other.
 5. The position sensor according toclaim 1, wherein the first resonator exhibits resonance in response to afirst range of frequencies about a first resonant frequency and thesecond resonator exhibits resonance in response to a second range offrequencies about a second resonant frequency which is different fromthe first resonant frequency, the first and second ranges overlapping,wherein the excitation generator is operable to generate an excitationsignal having a frequency component which induces the first and secondresonant signals in the first and second resonators respectively.
 6. Theposition sensor according to claim 1, wherein the first phase shift isdifferent from the second phase shift by one quarter of a cycle.
 7. Theposition sensor according to claim 6, wherein the analyser is operableto measure a phase of a signal formed by a weighted combination of thefirst and second resonant signals.
 8. The position sensor according toclaim 7, wherein the analyser is operable to generate a second signal ata frequency different from that of the excitation signal, and to mix thesecond signal with the signal formed by a weighted combination of thefirst and second resonant signals to generate a third signal having afrequency component equal to the difference between the frequency of theexcitation signal and that of the second signal, and to determine thesaid value from the phase of the third signal.
 9. The position sensoraccording to claim 1, wherein the first and second members arerelatively movable along a rectilinear direction.
 10. The positionsensor according to claim 1, wherein the excitation winding is formed bya conductive track on a planar substrate.
 11. The position sensoraccording to claim 10, wherein the planar substrate is a printed circuitboard.
 12. The position sensor according to claim 10, wherein theexcitation winding effectively comprises a plurality of loops arrangedso that current flowing through the excitation winding flows around atleast one of the loops in an opposite direction to at least one other ofthe loops.
 13. The position sensor according to claim 1, wherein atleast one of said first and second resonators comprises a passiveresonant circuit.
 14. The position sensor according to claim 1, whereinat least one of said first and second resonators comprises an amplifierfor amplifying the power of a signal induced in the resonator.
 15. Theposition sensor according to any preceding claim 1, wherein the firstand second resonators comprise respective conductive tracks formed on aplanar substrate.
 16. The position sensor according to claim 15, whereinthe planar substrate is a printed circuit board.
 17. The position sensoraccording to claim 2, wherein the sensor winding is formed by aconductive track on a planar substrate.
 18. The position sensoraccording to claim 17, wherein the sensor winding is formed on a printedcircuit board.
 19. The position sensor according to claim 17, whereinthe sensor winding is formed in a single loop.
 20. The position sensoraccording to any preceding claim 1, wherein the excitation signalcomprises a sinusoidal component at 1 MHz.